The present invention relates to energy storage means and, more particularly, to novel means and method for operation of an energy storage element to maintain the voltage of a load above a required non-zero minimum voltage.
It is known that some types of electrical power-consuming loads, such as an arc discharge lamp and the like, require a DC load-energizing voltage having a magnitude which never decreases below a certain "holding" value; a load voltage less than the holding voltage can cause the load to cease operation in a desired mode, with potentially deleterious results. This minimum voltage requirement, in turn, places restraints upon both the manner of energy storage prior to the load and the method of operation of the energy storing means. Considering an arc discharge lamp load as an example, it is known that a metal halide lamp exhibits several modes of operation. In particular, most such discharge lamps pass first through a high-voltage breakdown mode and then through a glow mode, prior to commencing operation in the desired steady-state arc-discharge mode. If the arc current through the discharge lamp is interrupted for a sufficiently long time, which may be as short as 0.1 milliseconds, the discharge may be required to again pass through the glow mode, or possibly even through both the high-voltage breakdown mode and the glow mode, before arc mode operation is again possible. Additionally, discharge current interruption will allow the discharge impedance to increase, so that higher voltages must be applied to maintain the arc, even if the discharge is not returned to the glow mode. Therefore, a lamp-energizing ballast will preferably receive an operating potential which never falls below that potential at which the ballast can no longer properly energize the lamp and allows the lamp discharge to fall out of the desired arc mode of operation. In a co-pending U.S. application Ser. No. 659,754, filed Oct. 11, l984, a ballast for an arc discharge lamp, operating from the direct current available from a full-wave-rectified AC source, utilized a capacitor for supplying lamp-energizing current when the rectified source voltage magnitude was less than the lamp load holding voltage. While this form of DC lamp-load ballast allowed a large value capacitance (e.g. about 50 microfarad) to be reduced to a lower value (e.g. about 10 microfarads), the capacitor is still of a size requiring the use of an electrolytic type, which is generally larger than desired for inclusion in a ballast compartment physically attached to the lamp itself, and also prevents the cost of the final lamp product from being reduced to a desired value. It is therefore highly desirable to provide both an energy storage capacitor-containing apparatus which can be used as a pre-ballast to supply energy to the actual ballast and thence to the lamp load, and a method for operating the pre-ballast apparatus to reduce the size of the energy storage capacitor when utilized with a controlled ballast energizing a load requiring a load voltage maintained greater than some minimum value, such as occurs with a ballasted metal-halide-type lamp load and the like.